Method and apparatus for electrostatic separation of fine powders



p 7. 1966 c. Y. J. BOUTEILLE 3,

METHOD AND APPARATUS FOR ELECTROSTATIC SEPARATION OF FINE POWDERS Filed May 15, 1962 -PUMP 12 VOLTAGE 9- SOURCE I FINE AVERAGE COARSE l PUMP Inventor (LY. I 'Bou-rElLLE Attorneys United States Patent 6 Claims. (a. 209-129 V The invention relates to an electrostatic method of treating fine powders, and more particularly to the classification of powder according to the size of its grains.

The invention principally relates to powders which are electrically conductive, i.e. which are not insulating, the powder being of metallic, mineral or organic origin, and of which the material has only a negligible vapour pressure under the conditions of treatment.

The invention consists essentially in charging (preferably positively) each particle at the moment at which it leaves the supply device of the electrostatic plant for operating the method and in subjecting the charged particles to an electrostatic field. The operation is preferably carried out under a vacuum.

As a general rule operations are carried out at a sulficiently low temperature for thermoionic emission from the particles to be negligible. However, in the case of very fine powders, although the potentials to be employed are relatively low (no more, in principle, than a few hundred volts), the electric field at the surface of the particle may reach several millions of volts/cm. and the cold emission of electrons discharges the particle. It is in order to avoid this undesirable phenomenon that preferably the material is at a positive potential and that preferably a vacuum is used as dielectric.

If a thin stream of positively charged powder is caused to flow between two parallel flat plates which are vertical and at different potentials, it will be noted on the base plane of the apparatus which is perpendicular to the plates that as the grains fall they are diverted toward the negative plate, the extent of the diversion of each grain being inversely proportional to the square of its mean radius, i.e., if particles of radius R reach a point 0 on the receiving plate and particles of radius R reach a point 0' separated from the point 0 by a distance D, a particle of radius R will arrive at a point 0" separated from the point 0 by a distance D', such that (K being a constant). This allows the desired granulometric fraction to be separated, the fine particles to be recovered and the particles which are too coarse to be sent back to the grinding process. If an observation port is made in the wall of the evacuated chamber it will be noticed that the paths of the particles are practically linear (FIG. 1). There seems to be no limitation to the process since, at the limit, one reaches a kind of mass spectrograph. The law of l/R shows that the process is very particularly indicated for the granulometric analysis of fine particles, less than a micron for example, since it is the more effective the finer the particle. In the case in which the powder has been negatively electrified, separation does not take place as predictably. The electrodes should be far enough apart to prevent the finest particles from touching and creating an electrostatic turbulence which is prejudicial to electrostatic separation. In this case it would be necessary to interrupt the powder supply, and to disconnect the source of potential in order to drop the fine particles outside the field. After a few seconds stoppage, operation may recommence.

Since it may not be necessary to have a law of distribution as great as 1/R it is also possible to take 3,275,139 Patented Sept. 27, 1966 ICC electrodes of a shape and arrangement such that this occurrence is produced only at higher potentials than for parallel plates (FIG. 2).

The combination, within the same chamber, of grinding and electrostatic separation allows a powder of given granulometry to be made since the desired portion is removed as it is obtained during continuous operation.

The voltage generator may comprise accumulator batteries in series since, for very fine powders of the order of 0.1 micron for example, the potentials necessary are only a few tens of volts and the output current is expressed in milliamperes. It may be seen that we are very far from the potentials to which electrostatic machines have accustomed us.

If very good precision in the granulometric distribution is desired, the regularity of the output of the powder will be closely watched and this output will be limited so that the space charge carried by the powder does not excessively disturb the distribution or spreading of the particles in the electrostatic field between the electrodes.

The method may quite naturally be applied to metallic powders, of which the electrical conductivity is of course excellent, but it may also be applied to other less conductive substances such as silicon carbide, metallic sulphides, ferrites, etc. The sulphides are particularly important since they may very easily be obtained by precipitation and hence already have great degree of fineness. Even in the case of molten sulphides, shot-blasting, due to the friability of the sulphides, allows the electrostatic apparatus to be supplied as required after passing through a centrifugal sedimentation apparatus which separates all the grains smaller than a micron, for instance.

If it be desired to have particularly fine metallic powders, these latter are liable to be pyrophoric upon emerging and are thus not easy to handle. In this case, sulphides of the same granulometry are liable to be unstable. They are transformed into an air-stable oxide. This oxide will he reduced to the metallic state at the moment when they are used.

It follows that the method is particularly useful for the treatment of fine, and particularly ultramicroscopic powders, that it is not restricted to conductive powders since, by transformation of sulphides, for instance, there may be obtained electrically insulating oxides as fine as desired (paint pigments).

The invention will now be further described with reference to the accompanying drawing, given by way of example, in which FIGURES 1 and 2 are both diagrammatic views in vertical section of two installations for carrying out the process in accordance with the invention.

Referring to FIGURE 1, the installation comprises a volume 3 under a high vacuum, this volume being defined by two fiat vertical electrodes 4 and 5 and being located within an enclosure 1 evacuated by pump means 13. The electrode 4 is extended upwards to form an inclined ramp 6 upon which a stream of particles 7 falls. The electrode 4 is connected to the positive pole and the electrode 5 to the negative pole of a voltage source 12 supplying a voltage so as to form an electrostatic field in the volume 3 between the electrodes. A receiving tank 8 is arranged beneath the electrodes 4 and 5 within the enclosure, the particles accumulating in the tank in a heap 9. Experiments have shown, as mentioned above, that the particles 7 are positively electrified by contact with part 6. The particles tend to retain this charge since the vacuum inhibits leakage and when theypenetrate into the volume 3 they are deflected along trajectories included in a cascade 10-11, the trajectory 10 correspending to the smallest particles while the trajectory 11 corresponds to the largest particles.

Referring now to FIGURE 2, corresponding parts have been given the same reference numerals. However the negative electrode 5 now has the shape of a cylinder with a horizontal axis, determining with the electrode 4 a non-uniform field in the direction of a horizontal plane, so that the particles are not now distributed in accordance with the law distributing them in inverse proportion to the square of the mean radius, but in accordance with another, more empirical law.

I claim: I 1. Apparatus for the electrostatic sorting of fine elec trically conducting particles of the order of one micron and smaller in size, comprising an enclosure, means for evacuating said enclosure, a first electrode in said enclosure and being extended at its upper end by a ramp positioned to receive the particles to be sorted, a second electrode in said enclosure spaced from said first electrode, means for applying avoltage between said electrodes such that said first electrode is positive with respect to said second electrode, and means for collecting the sorted particles.

2. Apparatus for the electrostatic sorting of fine electrically conducting particles of the order of one micron and smaller in size, comprisingan enclosure, means for evacuating said enclosure, a fiat vertical first electrode bounding one of the ends of said enclosure and being extended at its upper end by a ramp positioned to receive the particles tobe sorted, a second electrode in said enclosure spaced from said first electrode, means for applying a voltage between said electrodes such that said first electrode is positive with respect to said second electrode,

means for feeding said particles onto said ramp, and

-means for collecting the sorted particles.

3. The method of electrostatically sorting fine electrically conductive particles of the order of one micron and smaller in size, which method comprises the steps of main: taining in an evacuated enclosure between positive and negative electrodes an electrostatic field which permeates a volume between said electrodes which has a substantial vertical dimension, causing said particles to become positively charged, and feeding them under the influence of gravity into said volume, whereby their trajectory within said volume is influenced by said electrostatic field in accordance with their size.

4. The method claimed in claim 3 according to .which said particles are charged by feeding them .in a stream onto said positive electrode.

5. The method claimed in claim 3 according to which said particles are fed in a fiat stream onto saidpo'sitive electrode.

6. Apparatus for the. electrostatic sorting of fine elec-- trically conductive particles of the order of one micron and smaller in size, said apparatus comprising an enclosure, means for evacuating said enclosure, a first electrode in said enclosure, a second electrode in'said enclosure spaced from said first electrode, means for applying a voltage between said electrodes such said first electrode is positive with respect to said second electrode, and means for feeding said particles into said enclosure onto said first electrode.

References Cited by the Examiner UNITED STATES PATENTS Hedberg 209- OTHER REFERENCES Electrostatic Separation of Mixed Granular Solids,

by O. C. Ralston, pp. -146, Elsevier Publishing Company, 1961.

- FRANK W. LU'ITER, Primary Examiner.

RICHARD A. OLEARY, I. SPENCER OVERHOL SER, Examiners.

H. F. PEPPER, Assistant Examiner 

1. APPARATUS FOR THE ELECTROSTATIC SORTING OF FINE ELECTRICALLY CONDUCTING PARTICLES OF THE ORDER OF ONE MICRON AND SMALLER IN SIZE, COMPRISING AN ENCLOSURE, MEANS FOR EVACUATING SAID ENCLOSURE, A FIRST ELECTRODE IN SAID ENCLOSURE AND BEING EXTENDED AT ITS UPPER END BY A RAMP POSITIONED TO RECEIVE THE PARTICLES TO BE SORTED, A SECOND ELECTRODE IN SAID ENCLOSURE SPACED FROM SAID FIRST ELECTRODE, MEANS FOR APPLYING A VOLTAGE BETWEEN SAID ELECTRODES SUCH THAT SAID FIRST ELECTRODE IS POSITIVE WITH RESPECT TO SAID SECOND ELECTRODE, AND MEANS FOR COLLECTING THE SORTED PARTICLES. 